Why this day exists

Yesterday's program printed text chosen while you wrote it.

Useful programs must remember changing facts: how many hikers arrived, how much water remains, whether a trail is open, and which label a person typed.

Those facts can look similar in the terminal while behaving very differently in a calculation.

The difference appears as soon as a whole-number calculation needs a fractional answer.

Choosing how a value is represented is therefore part of choosing whether the program can be correct.